Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 16

it was observed that 229 genes in these cell lines are maintained in repressed
state by catalytically inactive enzyme. The gene repression correlated with
loss of the active histone marks H3K4me2, H3K4me3, and H3K9Ac
through interaction between DNMT1 and lysine-specific demethylase 1
(LSD1). This study thus represents yet another example whereby DNMT1
can repress gene transcription in the absence of its methyltransferase activity.
3.4. Role of accessory and interacting proteins in DNMT1
methyltransferase function
In addition to the fundamental chemical mechanism of maintenance methyl-
ation that is shared by Arabidopsis and mammalian maintenance met-
hyltransferases, other important features are shared between plants and
mammals. This includes the essential role of UHRF1, a mammalian protein
that is highly homologous to an Arabidopsis protein. UHRF1 is the actual pro-
tein that “recognizes” hemimethylated CpG sites ( Bostick et al., 2007 ). It has
been suggested that changes in electronic structure of methylated cytosine
destabilizes the UHRF1-unbound state and stabilizes the UHRF1-bound
state ( Bianchi and Zangi, 2012 ). In this manner, UHRF1 can recognize a
hemimethylated template and assist in stabilizing an intermediate state in
which the unmethylated cytosine opposite a methylated CpG is rotated
out of the double helix. Once outside the double helix, the unmethylated
cytosine can be methylated by DNMT1. Coimmunoprecipitation studies
have shown that UHRF1, Tip60, HDAC1, and DNMT1 together form a
complex, suggesting that the coordinated functions of UHFR1 and DNMT1
are highly dependent on the local chromatin and chromatin remodeling fea-
tures, particularly the level of histone acetylation ( Achour et al., 2009 ).
Acetylation and ubiquitinylation are two modifications that subject
DNMT1 to proteasome-mediated degradation during the cell cycle
( Du et al., 2010 ; Fig. 1.6 ). Replication of fully methylated DNA results
in hemimethylated DNA and the process of maintenance methylation acting
on the hemimethylated intermediate involves DNMT1 in a complex with
UHRF1, Tip60, histone deacetylase 1, and the deubiquitinase HAUSP at
the replication forks. Following maintenance methylation (establishment
of the methyl group on the newly replicated daughter strand), HAUSP
leaves the DNMT1 complex resulting in acetylation of DNMT1 and sub-
sequent ubiquitinylation so that DNMT1 is degraded at the end of replica-
tion. Degradation of DNMT1 is prevented by HDAC1 and HAUSP.
The histone acetyltransferase activity of Tip60 is responsible for H2AK5
acetylation at active genes. Although downregulation of UHRF1 resulted in